Many devices utilize a serial RS232 port as a serial interface. RS232 describes the standard for serial data communication between PCs and peripheral devices, such as modems, mice etc. and between PCs. There are two kinds of devices (from the viewpoint of RS232 communication): DTE and DCE. DTE stands for Data Terminal Equipment and means the “terminal devices”, i.e. the PC or a simple terminal and DCE stands for Data Communication Equipment, i.e. modems, mobile phones etc.
For example, network devices, such as routing platforms, often utilize a serial RS232 port to interface with the console, which is a DTE. To be able to support a backup WAN link usually requires the use of an auxiliary port to connect with an external modem which is a DCE.
A set of signals are defined for the serial RS232 port. The important signals in the present context are DTR (Date Terminal Ready) asserted by the console; DSR (Data Set Ready) asserted by the modem in answer to DTR; RTS (Request to Send) asserted by the sender; and, CTS (Clear to Send) asserted by receiver in answer to RTS.
To establish a DTE to DTE connection a special cable is used where RTS and CTS are electrically connected.
At the initialization of a connection the DTR and DSR lines are utilized as follows:
DTR/DSR: These control lines assure that there are really two devices present which want to communicate with each other: First, the DTE asserts DTR, then the DCE answers asserting DSR.
During a data transfer the RTS and CTS lines are used as follows:
RTS/CTS: These are the actual flow control lines: The sender asserts RTS (ready to send), the receiver answers asserting CTS (clear to send). If the receiver needs a pause (buffer overflow, for example), it de-asserts CTS, so the sender knows it has to wait with further transmission until the receiver has asserted CTS. This way it is assured that no data is lost.
In some systems, an alternate way of accessing the routing platform command line interface is through telnet. If for whatever reason, if the telnet option fails, the user is required to perform recovery techniques to access the router configuration.
Because port space on a device is usually a scarce resource, there is a need to use a single RS232 port for console and auxiliary port functions. In current devices, a user needs to manually configure the RS232 port to function as either the console or the auxiliary mode. Once the RS232 port has been configured to the auxiliary mode, a user is locked out of the router CLI command line interface. Thus, if the single serial RS232 port has been configured to support a modem and both the modem and telnet links are non-functional the user can be prevented from reconfiguring the routing platform.
Accordingly, an improved serial interface for network devices and other devices is required.